In order of fastest absorbtion....IV, IM, and sub-Q
I'd inject IM, IV is not worth it if you get a dirty product. Unless you are 100% for sure you have PURE HGH do not inject IV. I've done it before, and to be honest it's really not that much different from IM. You get sort've a euphoric "high" which feels good, but I didn't notice any difference in the potency.